Origin of the Earth

The Earth

As a preparation for the systematic study of rocks, it will be useful to look at the earth as a whole, inquiring into its origin, the changes that it has undergone and is still undergoing, and the nature and causes of the great variety of surface features.

Origin of the Earth

From comparison with the sun and other heavenly bodies that are known to be fiery globes, it is inferred that the earth began in the same manner, reaching its present condition by cooling until a solid crust was formed. That the interior of the earth is hot, at least in places, is shown by hot springs, volcanoes, lava flows, and by the steady rise in temperature with increasing depths in mines, tunnels and boreholes.

Structure of the Earth

The earth’s crust is made up of rocks, more or less covered with loose material that has been formed by the decay and breaking up of rocks. This loose crust consists of soil, subsoil, boulders, sand, clay, gravel, etc. Water is present everywhere, down to a considerable depth.

The solid crust is supposed to be ten or twelve miles thick. Beneath that, the temperature is calculated to be high enough to keep the rock- matter, or rock magma, in the melted condition, and at a still greater depth, the temperature is high enough for the rock magma to be gaseous. But the enormous pressure due to gravitation keeps the whole globe as rigid as steel.

Weight of the Earth

The earth weighs about twice as much as it would, if the specific gravity of the whole globe (5.57) were the same as that of the known rocks (averaging about 2.7). This may be explained by supposing that heavier materials, such as iron, occupy the central part of the earth. This idea agrees with the well-known fact that meteors, which may fairly be assumed to be samples representing the composition of heavenly bodies, are made up partly of rock materials and partly of iron, etc. But, on the other hand, the greater density may possibly be explained by the enormous pressures existing within the earth.

Surface of the Earth

The appearance of the surface of different parts of Canada may be taken as representing that of the whole earth. Southern Ontario is flat, with rock outcrops, which are composed of flat-lying layers. Southeastern Quebec and the Maritime Provinces show a surface that is partly flat, as in Prince Edward Island and in much of New Brunswick, and partly hilly, with bare rock exposed, as in much of Nova Scotia, Quebec, and northern New Brunswick.
The northern parts of Quebec, Ontario, and Manitoba present a varied surface, low rounded hills, valleys, lakes, muskegs, swift rivers, numerous falls and rapids, and many rock exposures—some stratified, more showing no layers—the great preCambrian “shield,” enclosing Hudson Bay like a V, the point of which extends down to the St. Lawrence, below Kingston. The prairie provinces, Manitoba, Saskatchewan, and Alberta, are level country, the greater part of which shows no rock at the surface. In startling contrast is the “sea of mountains” of British Columbia, with jagged peaks of bare rock, steep valleys, torrential streams, and glaciers.

There are a number of things to be noticed and explained concerning the loose material that covers the solid rock. In most parts of Canada, there are to be seen pieces of rock on top of the soil, or partly buried; they are often rounded, but are sometimes angular. When resting upon solid rock, they are usually found to be of a different kind from the rock upon which they lie; and it is often known that there is no solid rock nearby of the same kind as these boulders or erratics.

In some places, both large and small boulders are gathered in heaps or ridges, sometimes covering large extents of country; these are called moraines, eskers, etc. The loose material is often sand, which may be in distinct layers, i. e., stratified. If the grains of sand are carefully examined, they will be seen to be mostly quartz, with here and there a grain of mica, magnetite, ilmenite, garnet, etc. The sand grains may be round or nearly so, like sand that is well washed and worn on a lake or sea beach; or the grains may be more angular, sharp sand, the kind that is best for mortar. Beds of clay are sometimes clean and stratified. In other places clay is found mixed with boulders and not distinctly in layers, the so called boulder clay. Such irregular mixtures of clay, sand, gravel, and boulders are called drift.

There are also to be noted heaps, scattered pieces, or large areas of broken rock of the same kind as the solid rock beneath them or quite near them. Often the broken rock forms a sloping pile, or talus, at the foot of a cliff or steep hillside.

In some places there is no loose covering, and the solid rock appears at the surface as an outcrop. It is important not to mistake for an outcrop a very large boulder showing above the soil.